You can lock iPads on an app using access restrictions built into iOS. Once activated, it requires triple-clicking the home button to enable and disable this 'Guided Access' mode. Guided Access mode, when activated, disables the home button, and you can specify a bunch of other features or areas of the screen that are off limits.
Yup. Being doing that since my son was born. He has my old iPhone 3gs as serviceless cell phone. He loves having a real camera and pretending to be like Mom and Dad.
There are some decent Apps:
The LEGO DUPLO Train is a favorite
He likes Build-a-Bridge (not sure what the real name is)
Whatever the airport game is where you land the planes/helicopters and make them crash, I mean keep them from hitting